1. Technical Field
The present invention relates to a control device for motor driving, for example, a motor control device using a capacitor of small capacitance.
2. Description of Related Art
In recent years, efforts have been made to considerably reduce the capacitance of a smoothing capacitor to thereby downsize a motor control device.
For example, according to Japanese Patent Laying-Open No. 2002-51589, the capacitance of a smoothing capacitor is considerably reduced to intentionally cause a ripple of frequency twice that of the power supply to occur in a DC voltage, thereby achieving an improved input current waveform and a high power factor with a simple system.
Further, the capacity of a coil to be provided between an AC power supply and a rectifying circuit can be considerably reduced or eliminated.
On the other hand, reducing the capacitance of a smoothing capacitor causes the voltage to rise rapidly when the smoothing capacitor absorbs a large regenerative energy generated at a halt of a motor. The over-voltage may damage a switching device for an inverter in a motor control device.
Therefore, there has been proposed a system for protecting each driving device such as a switching device from a damage caused by the over-voltage of the regenerative energy generated in a motor even if the capacitance of a smoothing capacitor is reduced. For example, Japanese Patent Laying-Open No. 2005-198377 discloses such a system.
FIG. 5 is a block diagram showing a motor control device of the aforementioned publication.
As shown in FIG. 5, the motor control device has a connection using a power supplied by an AC power supply 1 as an input to a rectifying circuit 2 and using an output of rectifying circuit 2 as an input to a smoothing capacitor 3 of about 1/100 of the capacitance of a conventional smoothing capacitor. An inverter 4 includes six transistors (5u-5z) and diodes (6u-6z), connected in inverse relative to the transistors, in a three-phase bridge connection, and is connected in parallel with capacitor 3.
A motor 7 is connected using an output of inverter 4 as an input. A control circuit 8 drives the gates of transistors (5u-5z) of inverter 4. A power supply circuit 9 supplies electric power to control circuit 8.
A capacitor 10 is connected in parallel with smoothing capacitor 3 through a diode 11 serving as a rectifying circuit, and a load 12 is further connected in parallel with capacitor 10. Thus, since the regenerative energy of the motor is absorbed in capacitor 3 and capacitor 10, and energy is consumed in load 12 before the voltage exceeds a breakdown voltage, each driving device can be protected from being damaged.